Pass-band filter having electronically adjustable midfrequency

ABSTRACT

A pass-band comb filter utilizes a series of strips and varactors in combination to form parallel resonant circuits at a particular frequency. The pass-band midfrequency can be varied in response to the bias voltage applied to the varactors. Additional capacitors and input and output strips are used to compensate for the insertion attenuation and for matching source and load impedance.

`United States Patent t191 Petitjean et al.

[451 June 10, 1975 PASS-BAND FILTER HAVING ELECTRONICALLY ADJUSTABLE MIDFREQUENCY Inventors: Christian H. Petitjean,

Fontenay-auxRoses; Maurice E. L. Marchand, Sevran; Marcel Denis, Paris, all of France Assignee: International Standard Electric Corporation, New York, N.Y.

Filed: May 6, 1974 Appl. No.: 467,045

Foreign Application Priority Data May 18, 1975 France 73.18092 U.S. Cl 333/73 R; 333/73 W; 333/84 R;

' 333/98 R Int. Cl H01p l/20; HOlp 7/10 Field of Search 333/73 R, 73 C, 84 R, 84 M, 333/70 S, 70 R, 97 R, 98 R; 334/15 [56] References Cited UNITED STATES PATENTS 3,348,173 lO/l967 Matthaei et al 333/84 X 3,391,356 7/1968 Bolljahn et al 333/84 X 3,539,953 10/1970 Jones 333/73 3,649,937 3/1972 Carlson 334/15 Primary Examiner-,lames W. Lawrence Assistant Examiner-Marvin Nussbaum Attorney, Agent, or Firm-John T. Ol-lalloran; Menotti J. Lombardi; Richard A. Menelly [5 7 ABSTRACT 4 Claims, 6 Drawing Figures Y N N. N

l u# mt, a e., S P D A e i ."3

s u@ "c y S Y i P10 PATENT-ED JUN l 0 i975 SHEET BACKGROUND OF THE INVENTION This present invention relates to pass band filters using varactors and tuning capacitors. More particularly, it relates to pass band comb filters which are particularly usable within the GHz frequency range. At GHz frequencies lumped constant circuits comprising capacitors and self-inductances are difficult to utilize due to their low O. Filters using wave guides as supports may produce satisfactory results while leading to cumbersome designs. Structures better suited for utilization at these frequencies have been sought; for example, having "sandwich" and "triplate" type structures. One type of filter utilizing these structures is called "comb filter." lt comprises parallel strips located within a very flat casing, each strip having one of its ends secured toa 4casing sidewall. The strip length is approximately equal to )to/8, being the wavelength corresponding to the pass band midfrequency in the dielectric medium. The strips, each being associated with a capacitor which connects its other end to the casing, form parallel resonant circuits at the design frequency. Strip intercoupling is produced by) electromagnetic leakage fields. Two non-resonant terminal strips provide filter coupling to the source and the load, respectively.

Typically these filters have several important and significant advantages. Since strips are secured to a casing sidewall, utilization of dielectric supports is avoided. The casing sizes are reduced since a filter with midfrequency of l GHZ has a width of about 5 cm and an inside length necessary for locating four resonant strips and two input and output matching strips of about 6 cm. The strips being short and solidly secured to the casing sidewall reduce vibration effects. From a technical point of view, further advantages may be indicated. The second harmonics are `suppressed because the strips are, for those harmonics` resonant circuits coupled by very high impedances. When the strip length is about )to/8, the attenuation curve slope is considerably increased beyond upper frequency of the pass band. Resonant circuit intercoupling can easily be adjusted by varying the strip spacing.

These qualities enable comb filters to be used in radio-navigation equipment. To be more specific, certain modern applications will be mentioned as interrogators in TACAN systems, wherein transmitters and receivers may either transmit or receive on a great number of close channels; for example` 240 channels within a range from 960 MHz to 1,200 MHz, wherein transmission and reception frequencies in a channel are spaced by an intermediate frequency of 60 MHz.

To avoid image frequency effects preselectors have to be introduced which are in the form of pass band filters having attenuation curves rapidly climbing outside the pass band.

The minimum number of those filters is equal to four when filter switching is made by known processes. However, with four filters it is conceivable that for channels located in a border area between two filters `image frequency effects are not suppressed because the filter attenuation curve slope in the proximity of the pass band is not infinite. To avoid this drawback the number of filters must be increased. For instance, six

filters may be provided instead of four, each having a pass band of 40 MHz, but in this case size becomes prohibitive and filter switching problems become pronounced.

SUMMARY OF THE INVENTION One purpose of the present invention is to substitute a single filter for a number (p) of switchable adjacent filters of band width Afa, the single filter having the same band width Afm but having a midfrequency F(l which may be varied over all the range Fo p A12, by electronic means.

According to a feature of this invention fixed capaci tors associated with the resonant circuits are replaced by varactors or voltage variable capacitors so that pass band switching is performed by varying the bias applied to each varactor, the bias variation being either continuous or discrete.V

According to another feature of this invention, varactors have their own Q high enough and operate under high bias voltage V so as to reduce responsiveness dC/dV. Thus, filter preformances are further free from midfrequency jitter which could result from important capacitance variations due to temperature effect occuring when utilizing low voltage operated varactors.

According to this invention, the major interest of varactor utilization with high bias voltage is in relation with the fact that internal resistance ri serially connected with capacitance C decreases with C. ln such a manner unloaded Q1 for resonant circuits to the extent that losses have no other origin than resistance r.- is increasing with frequency.

In a comb filter using no-loss capacitors frequency band and insertion attenuation curve, both in pass band range and in attenuated range, are basically depending on a coupling factor which must be higher than 1 and is expressed by the ration wherein:

R is the source resistance from filter input or load resistance from filter output, Z0 is a coupling impedance between Athe strips as determined by their arrangement, and 0 is an angle proportional to the frequency which is 7r/4 when filter strip length is of 1ro/8. Qo being the Q of resonant circuits loaded by resistor R, coupling factor becomes:

The term Q,Q1 cot20 beinga function rapidly decreasing with frequency, u may be maintained constant when 6m that is frequency, is varying within cer` tain limits.

Therefore, according to a further feature of this ini vention, insertion loss curves remain identical for each position within frequency band AFO, due to a reduction of coupling impedance Za by a ration from 1 to Varactor utilization is accompanied by an absorption loss which effects low frequencies more than high frequencies in band AFU. The varactor assembly is controlled by a single bias and the capacitance addition for each resonant circuit is provided by a simple'settable air capacitor, such as a conventional capacitive screw. Due to such an arrangement only one voltage source may be used to control the varactors which may be utilized with high bias voltage` Such an arrangement is also of advantage when Chebishev performance filter is utilized wherein while resonant circuits have the same electric length they have different characteristics and then also different associated capacitances.

Other features of the present invention will appear more clearly from the following description of embodiments, the said description being made in conjunction with the accompanying drawings, wherein:

BRIEF DESCRIPTION OF TI-IE DRAWINGS `by an amplifier; and

FIG. 6 shows an alternative of the strip-varactor arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENT` FIG. 1 shows two cross-sectional views, along two normal planes, of a comb filter comprising four active strips and two additional strips for matching both source and load impedances.

Strips 1-1, 1-2, 1-3, and 1-4, each having one end secured to casing 3, are respectively terminated at the other ends by capacitors which are varactors -1, 5-2,

5-3 and 5-4 provided with biasing means which will be 2 2r cot 0 hereafter described. Free sides of varactors are RF short-circuited to casing 3.

Strips 2-1 and 2-2 respectively operate to match filter input and output with source and load impedances which are, for instance, impedances of coaxial cables 4-1 and 4-2.

Lengths of the six strips are equal to or less than )to/8, where )to is the pass band midfrequency.

Reference being made to the lower cross section shownin FIG. l, it is to be noted that each strip has a vcapacitance y1 per unit of length with respect to the casing, that capacitance y, varying with the thickness and width of the considered strip; Likely two adjacent strips are coupled by a capacitance y0 per unit of length, 'ya var-ying with strip thickness and spacing.

dielectric medium surrounding the strips. Likely two i adjacent strips constitute a `propagation line having a characteristic impedance Z which is substantially v,

higher than Z0, with 2,. l/yv. Example of a Pass Band Filterwith Two Active Strips embodiment of filter provided with two vactive strips will be considered.

The two active strips associated to the casing represent shortcircuited lines having impedance j Z0 tan 0, 0 being equal to wl/v where w is angular frequency 21rF and 0 is equal to or less than Frr/Ll.

Short-circuited lines are terminated at their free ends by capacitors C Parallel resonance is produced at frequency F when: i

lCmZO cot 0 I The two resonant circuits are coupled by a shortcircuited line of impedance:

j Z tan 0. Filter input and output resistors R arethose which are obtained in converting source'and load impedance p by means of the two side lines constituted by additional strips associated to the casing.

If Zl is characteristic impedance of the line formed by the first filter strip and the additional input strip and Z2 is characteristic impedance of the-line formed by additional input strip and the casing, it results therefrom R p (Z1/202. If it is anarrow band filter, R is necessarily large with respect to Z, as will be explained in the following, and, as p has about the same value as Z0, the additional input strip operates as an impedance booster transformer. The same consideration is valuable for additional output strip.

The filter, as shown in FIG. 2, brings between the two resistors R an insertion attenuation which may be easily computed according to the theory of quadripols.

In dB that insertion attenuation is written:

A dB lO login (1122 -l'l) where h2 is the ratio:

(1+ coto-CM., (A)

Formula A entirely defines the behavior of the filter in the pass band and out of the pass band;

Strips associated to the casing constitute propagation lines having a low characteristic impedance Z0, .e. Zo =1/y1v, vbeing electromagnetic wave velocity through Assuming that Rl/Z2 and Z/2Zo are relatively higher than 1, itis to be noted that poles of function h2 correspond to angular frequencies w between the two frequencies w, and (u2 for which cot 6, Cwl-Z O and w1 being the natural angular frequency for each of the resonators and m2 being a frequency symmetrical of to, with respect to band` midfrequency m0 wo being defined by Y and then g lt is still to be noted that h2 has Yeros for the two frequencieswl' and ing', difference Aat, 00.2"- w1' being written 2 tan l 6 cot 0), tan 0 where hinter I/Q (14"- y s 2u The same insertion attenuation appears again for frequencies which are respectively lower and upper than co2' and w1'.

. conventionally defines the filter bandwidth.

tion attenuation at frequencies w", and m2, i.e.

= ingwu a 42) Under w1 and above m2 attenuation rapidly increases in accordance with a law only depending on u.

Also to be noted that. for any assumption, when angle 9 approaches '1r/2, in other words when lines become naturally resonant (quarter wave lines), Am, increases to a and A(nl is reduced to 0. The pass band filter becomes a band rejection filter. l

Accordingly use of strips having length equal to or less than )to/Slis'justifled, length 'of )to/Sienabling 2nd harmonics to be removed, which often is technically wished.

Up to now assumption has been made that'filter components would correspond` topure reactances. Typically there are losses and, particularly when using varactors, losses are basicallydue tointernal resistance r1 in series `witheach varactor capacitance. s

ln a `general mannerv if Ithe 4two resonators are lossy they.have`y theirl own finite Q. Each resistor R is to be considered as mounted in parallel with a Vresistance ZQ tan 0. andURfmust be substituted for in (A), (B), (C) and (D), with Should only insertion attenuation is considered it is to be noted that the same characteristics may be find by reducing Z when Q is kno vn. t

An increase of bandwidth Aw, may even be recognized due to the factor (l -i- R/ZnQcot 0.,).

But absorption attenuation must also be taken into account since it indeed results from series resistance of varactors. In the case of two varactors, absorption attenuation is:

A': 2() lOgiu l l -iiQZ lt affects in a same manner any frequency does to F0.

When, according to this invention, midfrequency Fu is varied by varying biases applied to varactors, it results therefrom variations of capacitance C and Q. Without special care, A t-l, Aww and Am are varying with midfrequency F over the whole electronic tuned band: AR, pAf. t

Indeed a purpose of this invention is to define a relation between varactor characteristics and comb filter characteristics so as to reduce to minimum variations of the above mentioned quantities over electronic tuned band.

ln any assumption it will be selected a varactor having a relatively high Q so as to reduce A'.

ln a varactor, the width of the neutral area is given from equation:

lB being equal toHlz for alloyed junctions and to l/a for deep diffused junctions.` V measures negative bias voltage and v negative contact'voltage, v being of about Varactor capacitance C is given from equation:

Se Se C k(V+I/) S being junction area and being electric permittivity of the utilized semiconductor material.

It immediately appears that `the higher is V the smaller is dC/dV. According to this invention varactor is operated under high voltage V so as to provide filter operation stability and to make it particularly not responsive to temperature effects which affect v.

Another advantage, which is probably the more important advantage in operating at high voltage V will now be described.

VaractorY has the same behaviour as a capacitance in series with a resistance, the whole being shunted by a high resistance Rp which symbolizes all the other varacv tor losses. For a capacitance C corresponding to bias V,

Q is, for the varactor:

Q2 is the limit ofthe varactor when r,- is reduced to Therefore coupling factor becomes:

Absorption attenuation is;

l i R Rr A =201ogm (1 K+ Tem 6.,)

Resistance ri'is written:

pa r,

where p is resistivityin base region and e2 is base re gion thickness for a very low value V. When V .increases, el also increases and, as a limit, when bias Vr is of about 150 V, close to breakdown voltage, (e2 4el) becomes null as well as ri.' i

With bias Vr capacitance C has the'value Cr and r,- may then be written:

e C C r,= "C u- C1=f.. u g

Considering couplingv factor l R cot 0 u 2,/ R Rr it may be ma'de constant over band AE, it' u is mini mum for the value of '0., corresponding to maximum value Fc of band AF.

The function is minimum when 1y/d0, o.

Calculation` of that derivative, taking into account the law of r,- versus C, leads to the following condition, for frequency Ff:

i 1+ datlm where (6 0) varies with 90:

, l y (6") l 0 (tan 6 +cot 0,.) y

which varies from 1.76 to 1.64 when 0 goes from 1r/5 to 1r/4.

ln the following assumption will be made thatat frequency F,., 0o fr/4'and thus qb (1%); 1.64.

Attenuation A,,, at'frequency Fc, isz

A',, 2() logl (l At the other end of the pass band F,. AF4,

Assumption being forinstance made that alloyed structure typevaractor is selected, with a maximum bias value of `100 V, C/Cr is equal to 1.2-2 and Cr/(CC`r) is equal to 4,5. l

Selecting a relative band 1"/Fv 0.1 and neglecting losses caused by 1 .-l- R/Rc), it results Those value's'may be considered'as excessive. Therefore in most cases, the'varactor willbe only assigned vto provide capacitance variation over the band, the addition being provided by'a verylow-loss air-typefixed cvapacitor hvinga xed capacitor C ,L

' 1n such conditions, calculation of derivative L:1y/d0 leads to a new equation: 1 l

With the assumption that (CflC)/C 3, the previous C,+ C l (E) C l Cr C[+ C C-C.. C

lf pass band filter must have a relative electronic numerical example gives: l5 tuned band of 2O percent, A'Wu may be reduced from l 3.4 down to 1.8 dB by sweeping the band AF in two A,, :0.4 dB half-bands AF/2, varactors being utilized, as hereafter ,4 ",M 2 dB mentioned, first with a rst capacitance C f' in the upper half-band, then with a second capacitance Cf" in the As soon as maximum bias V, producing capacilower half-band, C," being larger than Cf'. tance C and minimum bias V,, producing capacitance l t (C AC) Corresponding to frequency Fc AF" have Up o now assumption has been made 1n calculations I t I i' that the varactor could be selected 1n such a manner been selected, A and A m as well as ((fl-C )/C are that u, is Constant over band AF practically determined as also are AC and (Cf-l-C) as a 25 f" function of Z. However considering the relation marking u conlndeed it successively results: stant, it appears that:

7lr (C,+ C) (0 con 9 with G), T

1r A 1"" l A C= T+ l (C,+ C) F 2.57 (C,+ C)

A C+C V.,., "2 G+ F C.

C 2'57" C r; +1 CC F3 Vm i Thus C 0.39 A F" [E Vm," l i] (F) VIINIJ l Wnh Vmln *5 C "015 Al- .i Cr lfm addition C Cr 4.5

. i R it results A',, 2() log,ll (l T) 20 login (l F" i l 3.7 F-

AF.. A',.= A'mm 20 lug... (l 2 l The following Table l indicates values of A,,, A',, Rf; emma: l and Cf+C/C` for several values of AF/F,.. lt is assumed. Z" l l 64 Cr 231C.

i C-C, C that R/R.. 0. 55

that is, since 0 l/4 and Rr,/Z2 is much smaller than 60 l, substantially:

C, C-Cr CftC 11 12 and in replacing r,- by its value The value u with no unloaded conditionlosses may Vthen be given to u' by reducing Z' from l to l0 C A'-/20, that isby practically reducing the tuned strip ri: -Ci- (l C space interval in the saine ratio.

5 Considering (C) that gives Aw coefficient wo s( 0) l R Crt-C R/ZQ cot 0o) is proportional to 00 s(0) cot 0 which only varies by l percent in relative value when 0increases from ir/S to 'rr/4.

or in introducing the value of ra that measures the serial resistance when V is low:

R Considering back u' 7 l it appears that, when-using varactor at low values of V that is in using equation (F) and noting that for which r1 substantially constant, u is then stationary when Cr-r Vmln lll y v' C Vr t i v i i Rl', R

Z2 cot20= l 25 R Vm t 1, 0.65 ZZ, (Tl LL] (G) and when R Coefficient 0.65 results from the selection of 0 Ar-20 legw (1+ IE7-l iT/4. Actually coefficient variation is very small when (ilo oscillates about 11/4.

From (G), it appears that, if VMI, V and F/Fc are 1S hlghet than 6 dj determined, the ratio ro/Zaz is also defmed That simple consideration shows the interest of using he Z(I is known, Selection will be made from varactor highly biased varactors, particularly due to increase of catalogue to find one bringing a suitable r. It may also ri with capacitance C- be considered that resonator characteristic impedance E i f b d fi i Zo is tted to a given Value ro. xamp e o pass an iters inc uding more than two Reversely if r/Z2 is known as well as V- and VMI, 40 resonators relative band Fa/Fc is then defined If a third strip is inserted into between the two previit wiii be noted that utiiizatien ef deep diffused juneeus mentioned Strips Whieh constitutes with easing a tion type varactors would lead to replace exponent 1/2 Propagation line having a Characteristic impedance by exponent 1/3 in (G) Z0/2, associated to a varactor having a capacitance 2C, That simple consideration immediately shows liow althe result. iS: @three-element filter. Y loyed junction varactors are of interestfor the present Calculation Shows that in this case, ratio hs S aS a application. function of h2:

As varactor parameters are usually determined with respect to resonator parameters, as it has iust been mentioned, u' is from (E): 50

22,.' 2 hii h2 Z" Cot 9 El Z", COI CLUZJ R l l h3 thus has the same poles as h2 plus a third pole correl Z3, cot 9" R 1 55 sponding to w. A v

l R l C Cnc Frequency gap Az, in the present case the same as in l L64 C-C., C the first case, i.e.

Denominator terms that are vdue to unloaded condition losses may be evaluated as a function of A'(db) Z., R2 2 U2 and it results: A w-w.- 0 R 20,2 eet o.,- i)

65 e f R Amm' It is to be noted that, within the pass band, h3 reaches two equal maximum values producing a maximum in- Z'n' 20 band insertion attenuation of Other things being equal this attenuation is substantially lowerthan in the first case.

Here insertion attenuation at frequencies w1 and wz becomes Av logm(l u2) Thus it is substantially higher than in the first case. Finally frequency interval A w, wherein insertion loss is at the most equal to AW,J is:

lt appears that coupling factor u R/Zo' cot 0 determines the filter performances as in the case of a tworesonator filter.` lt is as the third resonator contributed only to reshape insertion attenuation curve.

Therefore the modified coupling factor u' is the same as in a two-resonator filter and insertion attenuation c urves are equalized in a same manner over the electronic tuned band F.

However losses in the third varactor increase absorption attenuation that becomes, taking into account that median resonator characteristic impedance is half of characteristic impedance of the two others:

Assumption being made that Q' Q, term R/Z l/Q l/Q) is equal to 0.28 and A',,- is of2 dB at the maximum frequency Ii. of band 1i, when F/Fc 0.2, and of 5 dB at the minimum frequency.

The same calculations may be made for a fourresonator filter, which leads to the same results with re gard to coupling factor u that remains equal to R/Z' cot 0b. v

Over the pass band maximum insertion attenuation is substantially lower than for a three-resonator filter. There are four zero-attenuation frequencies interdigitated with thre maximum attenuation frequencies.

Bandwidth Aw, is

A' 2() log",

At frequencies w, and wg'outside the pass band. insertion attenuation A Losses in the fourth varactor increase adsorption attenuation which becomes if assumption is made that fourth resonator characteristic impedance is equal to third resistor characteristic impedance:

2 i Q, cot 0,]

Assumption being made that Q' Q, maximum of term R/Z (l/Q -l- 2/Q,) is equal to 0.42 and A' ='3 dB.

Still considering a relative electronic tuned band AF/Fr percent, 0 being equal to 'i1/4 at the upper frequency and to 1r/5 at the lower frequency, A' varies from 3 dB to 6 dB. With a relatibe band wherein AF/FC 10 percent, in the same other conditions, A varies from 2 dB to 4 dB.

FIG. 3 shows attenuation curve shapes for a fourresonator filter covering six portions, that is for six balues of bias voltages applied to the varactors one elec tronic tuned band from FD :AFD/2 to Fo -l- APO/2. Over each position bandwidth Aan/21T is slightly wider than Af so as to obtain sufficient laps. The six insertion loss curves A are substantially similar to each other with a slight widening of the pass band to higher frequencies of electronic tuned band. In addition each of them is identical to that which may be built with air capacitors. Finally absorption attenuation A' associated to each curve decreases when frequency increases.

These filters can be conventional 'rr-filters with equal coupling impedances jZ' tna 90 between the resona` tors. Attenuation however does occur, particularly in the higher range. However, considerations based on the coupling factor u and methods to hold the coupling factor constant in spite of losses occurring in the varactors can be employed within the scope of this invention.

FIG. 4 shows a filter according to this invention that includes four resonant strips. One adjustable capacitor, such as 6-1, 6-2, 6-3 or 6-4, is respectively mounted in parallel with one varactor 5-1, 5-2, 5-3 or 5-4. Capacitors 6 may be simple screws supported by the casing, which are more or less closed to strips 1-1 to 1-4.

Bias source V is common to every varactor that it supplies through a protection resistor 8 and insulator passages 7-1 to 7-4 having high capacitances. Tuning capacitances associated respectively to the four resonators have different values. Due to parallel capacitors 671 to 6-4 the needed capacitance variation AC for covering the whole electronic tuned band AF., may be the same for the four resonators. Therefore the four varactors are utilized in the same conditions from a single bias source.

When a preselector, wherein a filter according to this invention is` used, is mounted before a mixer, the loss of 4a few dB due to attenuation A' reduces the receiver sensitivity. Thus there is a need to first amplify the received signal in a low noise factor amplifier before applying it to the mixer. The comb filter structure, usually having a low impedance, is very convenient by using a transistor amplifier that is located within the casing.

An amplifier preselection device according to this invention is shown in FIG. 5.

A filter 9 includes two strips 10-1 and 10-2 associated to varactors 12-1 and 12-2 parallel mounted with adjustable capacitors 13-1 and 13-2. Two matching strips 11-1 and 11-2 are respectively coupled to input coaxial cable 14-1 and output coaxial cable 14-2. Varactors 12-1 and 12-2 are biased at a suitable value V through a protection resistor 15 and a potentiometer 16 from an adjustable voltage source V'. The under arm of 16 may be short-circuited by means of a switch 17 so that varactors have highcapacitance when 17 is on.

Output 14-2 is coupled, via capacitor 19, to the base of transistor 18 having its collector coupled, via capacitor 20, to input 4-1 of a four-resonator filter similar to that shown in either FIG. l or FIG. 4. Transistor 18 is supplied from a subsidiary source 23 througha highcapacitance insulating passage 22 and a conventional R-L-C filter 21. 9, 18, 21 and the four-resonator filter may be located in the same casing.

Filter 9 has a pass bandwidth Af wider than fourresonator filter bandwidth Af and may be tuned in the same electronic tuned band AF. Maximum attenuation A of 9 in band AF., is, for instance, of 2 dB and fourresonator filter maximum attenuation may be of 4 dB. Amplifier 12 may have a gain of l0-15 dB with a noise factor of 4 dB. Applying equations giving total noise factor FT of a series of active and'passive quadripole networks, and assuming that proper noise for each of the two filters is of 3 dB while that of the mixer is offlO dB, it results as a function of the amplifier gain G:

that is an improvement ofthe receiver sensitivity by 2 db withG=l0 dB. l

The selection of a bandwidth Af' wider that Afa makes possible in a preselector associated to a transmitter-receiver to protect the mixer for transmission period. An automatic process switch 17 may be closed and untuned filter 9 no longer transmit signals from 14-1. Therefore the described circuits easily solve the problem of mixer protection.

As already mentioned, when desired electronic tuned bands are very wide, the above described devices may produce moderate results since varactor bias must vary over a large range and it is no longer possible to satisfy equalization conditions for coupling factor u' without greatly increasing attenuation A'. The device, as shown in FIG. 6, makes it possible to overcome such a drawback.

Varactor 25 may be coupled either t'opar'allel capacitor 26 or to assembly of the two capacitors 26 and 27 To do so varactor 25 is supplied through protection resistor 32 and insulating passage 30 from bias source V. Capacitor 27 has its under electrode connected to the top of strip 24, through PIN diode 28 that is supplied through a small surge inductor 29, an insulating passage 31 and a protection resistor 33 from another'bias source V2 which may have two values, either a positive value or null.

When source V2 has a positive value, diode 28 has a very low resistance and the two capacitors 26 and 27 are parallel mounted with varactor 25. When the value of V2 is null, resistor of 28 is very highv and only capacitor 26 is in parallel with varactor 25. The other filter varactors being arranged in the same manner, it is possible with only one varactor bias source V and only one bias source V2 supplying PIN diodes to cover band AF `with two bands AFo/Z. Such an arrangement may be used also in preselector-amplifier as shown in FIG. 5.

While the principles of the present invention have hereabove been described in relation with specific embodiments, it must be understood that the said description has only been made by way of example and does not limit the scope of this invention.

What is claimed is:

l. An improved midf'requency bandpass comb filter of the type having a casing consisting of`a pair of opposingside walls, a plurality ofparallel strips, and a number of first capacitors, each of said strips directly'co'nnectedat one end to one of the side walls, and serially connected at the other end to the other of saidvside walls by means of each of said first capacitors wherein the improvement comprises:

' a plurality of varactor diodes having first and second terminals, each of said varactor diodes in common connection with each of said first capacitors by -1 means of said first terminals; and l a first source of variable bias voltage, each of said varactor diodes 'in common` connectionI with` said first bias voltage by means of said second terminals, whereby the midfrequency is displaced inres'ponse to variations in bias voltage causing variations'in capacitance to occur therein said varactor diodes.

2. The bandpass comb filter of claim l further including a plurality of second capacitors having first and Isecond terminals, each of said secondcapacitors connected directly to the other of said side walls by means of said first terminals, and connected serially to lthe other' end of said strip by means of said'second terminals, and a plurality of Ypin diodes each of said pin diodes connected to said second capacitors by means of said second terminals, said filter further including a plurality of surge inductors eachof said surge inductors commonly connected between said second capacitors by means of said second terminals, and a second bias voltage source, said pin diodes incombinationwith said second bias voltage therebycontrolling electrical continuity between said second capacitors and said strips depending on bias voltage values applied thereto saidpin diodes. 3. The bandpass comb filter of claim-Z-further including first and` second protection resistors, said first protection resistor serially connected between said .first lbias voltage source and said plurality of varactor diodes, and saidsecond protection resistor serially connected between :said second biasvoltage source-.and said plurality of surge inductors. f

4. The bandpass comb filter of claim l wherein said plurality of strips comprise four metal strips equidistantly located from each other and wherein said plurality of varactor diodes comprise four varactor diodes each of said four varactor diodes coupled with each of said four strips, and wherein each of said varactor diodes i s connected in common with said first bias voltage source byy means lof foui-l electrical connecting lmembers extending through four insulated vpassages in the other of said side walls. 

1. An improved midfrequency bandpass comb filter of the type having a casing consisting of a pair of opposing side walls, a plurality of parallel strips, and a number of first capacitors, each of said strips directly connected at one end to one of the side walls, and serially connected at the other end to the other of said side walls by means of each of said first capacitors wherein the improvement comprises: a plurality of varactor diodes having first and second terminals, each of said varactor diodes in common connection with each of said first capacitors by means of said first terminals; and a first source of variable bias voltage, each of said varactor diodes in common connection with said first bias voltage by means of said second terminals, whereby the midfrequency is displaced in response to variations in bias voltage causing variations in capacitance to occur therein said varactor diodes.
 2. The bandpass comb filter of claim 1 further including a plurality of second capacitors having first and second terminals, each of said second capacitors connected directly to the other of said side walls by means of said first terminals, and connected serially to the other end of said strip by means of said second terminals, and a plurality of pin diodes each of said pin diodes connected to said second capacitors by means of said second terminals, said filter further including a plurality of surge inductors each of said surge inductors commonly connected between said second capacitors by means of said second terminals, and a second bias voltage source, said pin diodes in combination with said second bias voltage thereby controlling electrical continuity between said second capacitors and said strips depending on bias voltage values applied thereto said pin diodes.
 3. The bandpass comb filter of claim 2 further including first and second protection resistors, said first protection resistor serially connected between said first bias voltage source and said plurality of varactor diodes, and said second protection resistor serially connected between said second bias voltage source and said plurality of surge inductors.
 4. The bandpass comb filter of claim 1 wherein said plurality of strips comprise four metal strips equidistantly located from each other and wherein said plurality of varactor diodes comprise four varactor diodes each of said foUr varactor diodes coupled with each of said four strips, and wherein each of said varactor diodes is connected in common with said first bias voltage source by means of four electrical connecting members extending through four insulated passages in the other of said side walls. 